Apparatus for the manufacture of cascade image amplifiers



p 1962 N. A. SLARK 3,055,725

APPARATUS FOR THE MANUFACTURE OF CASCADE IMAGE AMPLIFIERS Original Filed Jan. 15, 1960 FIG. 3.

- l9 INvENToR 71W M M BY 8 United States Patent 3,055,725 APPARATUS FOR THE MANUFACTURE (BF ACADE WAGE AMPLIFIERS Norman Arthur Slarlr, Chelmsford, England Original application Jan. 15, 1960, Ser. No. 2,648.

Divided and this application Mar. 10, 1961, Ser. No.

4 Claims. (Cl. 316-31) This invention relates to the manufacture of cascade image amplifiers, that is to say to image amplifiers wherein an optical image to be amplified falls upon a photo cathode the electrons from which are accelerated and focussed upon a fluorescent screen to form thereon a stronger optical image the light from which produces photo-emission from another photo-cathode which in turn emits electrons which are accelerated and focussed upon a further fluorescent screen where a still stronger optical image is formed to produce light to activate still another photo-cathode and so on through a desired cascade of image amplifying stages, each consisting of a photocathode and associated fluorescent screen, until an image of desired intensity is obtained on a final fluorescent screen.

FIG. 1 of the accompanying drawings shows schematically a typical known cascade image amplifier of the socalled sandwich type. Deposited on the inside of one transparent end wall A of the envelope B of the tube is a photo-electric cathode C on which an optical image of a subject D (represented as an arrow) is focussed by means of a suitable lens system represented by a lens E. Electrons emitted from the cathode C are accelerated and focussed by uniform electric and magnetic fields (generated by means not shown) on a fluorescent layer F on a thin transparent membrane G extending across the tube envelope. On the other side of the membrane G is a photo-cathode H. There are several further such membranes G, G" and so on, spaced along the tube, each with a fluorescent layer F, F" on one side and a photo-cathode H, H on the other. On the inside of the window I at the other end of the tube envelope is a final fluorescent layer K. The applied electrical and magnetic fields are such as to cause the photo-electrons emitted from the cathode C to produce a stronger reproduction of the original optical image on the fluorescent screen F, the light from which causes photo-emission from the cathode H to occur and produce a still stronger optical image of the fluorescent screen F which in turn causes photo-emission from the cathode H to produce a still stronger image on the fluorescent screen F and so on until a bright image is produced on the final screen K.

Great difficulties are experienced in producing a really satisfactory tube as exemplified in FIGURE 1. One of the dithculties lies in the fact that the manufacturing processing of the cathodes involves heat treatment. If all cathodes are heat treated together it is almost if not quite impossible to subject them to the same rates of heating and cooling and non-uniformity results from this fact, some of the cathodes turning out to be inferior to others. If, however, the cathodes are heat treated separately, one after the other, when in the envelope, it is extremely difficult to prevent the heat treatment applied to one cathode from adversely affecting and even seriously damaging those which have already been treated. Moreover, caesium is introduced during the heat treatment of the cathodes and may attack and damage the fluorescent deposits and/or become deposited on the inside of the envelope walls and cause so-called dark current effects when the finished tube is put into operation. Moreover, the necessity of minimizing damage to fluorescent screens by caesium and the necessity, in tubes in which the cathodes are processed individually, of minimizing risk of damage to an already processed cathode when processing another cathode, involve that the successive membranes shall be spaced relatively far apart and this in turn involves expensive heavy and carefully adjusted magnetic field systems to ensure accurate focusing of the photoelectric emission from each cathode on the next fluorescent screen in the cascade.

The present invention seeks to overcome the foregoing defects and to provide apparatus for manufacture which will result in tubes having cathodes of practically uniformly good quality, and which can be made relatively small and compact, with only small spacing between the successive membranes, and with only small dark current. Indeed, it may be possible to employ the present invention to produce satisfactory tubes with the membranes so close together-spaced by only a few millimetresthat effective focusing can be obtained without any applied magnetic focusing fields at all.

According to the use of the apparatus of this invention a method of manufacturing a cascade image amplifier includes the steps of mounting a plurality of separate membranes on a carrier member in an evacuated space, mounting parts of the envelope of said amplifier in separated positions in said space, processing (including heat' treating) said membranes to form photo-electric cathodes thereon when insaid space, transferring desired processed cathodes from said carrier to predetermined positions in one of said envelope parts while still in said evacuated space, bringing the separated parts of the envelope together and sealing them to one another to produce a hermetically sealed closed envelope with the processed cathodes in position inside it, and releasing the vacuum and removing the sealed envelope from the space.

Preferably the membranes are processed one at a time, while still on the carrier member, by being brought in turn into a heat treating oven mounted in the evacuated space.

Preferably the carrier member is a movable member adapted to carry the membrane in such positions thereon that, on moving said member, said membranes are moved through a path which passes through the heat treating oven and through a space adjacent one of the separated parts of the envelope, said membranes being lightly held in said positions so that, when a membrane has been brought into said space adjacent one of the separated parts of the envelope, it can be readily transferred from its position in said carrier member to a position in said one envelope part.

Preferably said movable carrier member is also adapted to carry the other separated part of the envelope, said other part of the envelope also being moved through a path which passes through the heat treating oven and through said space adjacent said one part of the envelope, said other part of the envelope being so held in position that it may be readily transferred from its position in said carrier member to a position adjacent and in contact with said one part of the envelope.

Preferably the image amplifier comprises transparent membranes each with a cathode on one side and a fluorescent layer on the other, and these membranes are mounted each across a short tubular mount which is lightly held in position on the carrier member and is adapted to fit into a tubular member constituting one of the envelope parts.

According to a feature of the invention an apparatus for carrying out a method of manufacture as hereinbefore set forth comprises an evacuatable chamber with a re movable lid, a wheel mounted in said chamber and constituting a carrier member adapted lightly to hold a plurality of membranes and at least one envelope closureportion circle concentric with the wheel axis, an oven mounted in said chamber and so positioned and constructed that, on rotation of the wheel, said membranes and said closure portion carried thereby will pass in succession through said oven, means for rotating said wheel from outside said chamber, means in said chamber for holding a main envelope portion with a space adjacent one end thereof said space being so positioned that, on rotation of the wheel, said membranes and said closure portion carried thereby will pass in succession through said space, a bellows structure for permitting the main envelope portions to be reciprocated from outside the chamber to close and open said space and thereby transfer said membranes in turn and finally said closure portion in said space to the interior of the main envelope portion, and means permitting said envelope portions to be pressed tightly together to cold seal together metal seal members on the adjacent faces of the envelope portions.

FIG. 2 of the accompanying drawings depicts one illustrative embodiment of this invention. FIG. 3 shows, so far as is necessary, a preferred modification of the apparatus of FIG. 2.

Referring to FIG. 2 there is an evacuatable chamber comprising a helmet-like lid member 1 of stainless steel and a base plate 2 having a hole 3 through which the chamber can be pumped out via a pipe (not shown) fitted vacuum-tight to the hole. The lid member 1 is bolted to the base 2 with interposed packing pieces 4 to provide a vacuum-tight joint. The lid member has a number of inspection windows in the top and sides, one of these windows being shown at 5.

In the upper part of the chamber is a tubular envelope carrier 6 in which can be fitted the major portion 7 of the envelope of an image amplifier. The part 6 is longitudinally movable between guides 8 and is carried by a bellows structure 9 as shown to permit it to be moved longitudinally by means such as a hand wheel 10 and screwed rod 11 from outside the chamber. The envelope portion 7 is formed with an end copper or other suitable flange 7. The complete envelope consists of the portion 7 and a closure portion 12 which is provided with an end copper or similar flange 12 and is fitted into a fixed part 13 suitably mounted in the chamber. The envelope portions 7 and 12 are spaced apart opposite one another in such a manner that by moving the part *6 endwise the flanges 7 and 12' can be brought into contact, said flanges being made of such material that they cold weld to one another if appropriate pressure is applied. In the position shown in the drawing, however, there is a substantial gap between the adjacent ends of the envelope portions 7 and 12.

Also mounted inside the chamber is a heating oven 14 of any suitable type adapted to provide satisfactory heat treatment for processing a photo-cathode in accordance with known processing practice. Within this oven are provided the necessary evaporators or other means required to deposit the required photo-cathode materials.

Also mounted inside the chamber is a heating oven 14 of any suitable type adapted to provide satisfactory heat treatment for processing a photo-cathode in accordance with known processing practice. Within this oven are provided the necesary evaporators or other means required to deposit the required photo-cathode materials.

Membranes on which photo-cathodes are to be formed are carried on a rotatable conical wheel 15 which is rotatable on an axial shaft 16. The Wheel can be rotated from outside the chamber in any convenient manner, for example, through a magnetic drive operable from outside the chamber. So as not to complicate the drawing, the means for rotating the Wheel 15 are not shown.

The wheel carries a ring of membranes on which photocathodes are to be formed. Each photo-cathode is formed on one side of a transparent membrane 17 on the other side of which is a fluorescent deposit except for the membrane on which the first photo-cathode is to be formed, this membrane, of course, not having a fluorescent screen.

Each membrane is mounted across a short tubular glass mount 18 which is lightly held to the wheel by a spring holder (not shown). The mounts 18 are of suitable size to fit into the envelope portion 7. The oven 14, the wheel 15, the holders for the mounts 18, and the parts holding the envelope portions 7 and 12, are in such mutual relationship that by rotating the wheel 15, the mounts 18 will traverse a circular path which passes through the oven 14 and the space between the envelope portions 7 and 12 when those portions are in the positions shown in FIG- URE 2.

The apparatus is used as follows: The lid is taken off the base 2. A number of mounts 18 each with its membrane having a fluorescent screen on one side are placed in the spring holders on the wheel 15; an envelope portion 7 is put in position in the member 6 and an envelope closure portion 12 with an already prepared fluorescent screen on the inside of its window, is placed in the part 13. The lid 1 is then put into position on the base 2 and the chamber is pumped out and maintained in an evacuated condition. The wheel 15 is rotated through its magnetic drive to bring the membranes carried by it in turn and one at a time into the oven 14 where each is heat treated and processed in accordance with known practice to form a photo-cathode. When the photo-cathodes have been formed they can be examined for condition by inspection through one or other of the inspection windows in the chamber without removing them from the said chamber. The number of membranes carried by the wheel 15 is greater than the number of cathodes required in the complete tube so that the ones having the best photo-cathodes can be selected for putting in the tube and any bad one rejected. One of the selected cathodes is then brought into position between the envelope portions 7 and 12 by rotating the wheel. The hand wheel 10 is then operated to cause the part 6, with the envelope portion 7 inside it, to slide over the mount 18 which is in the space between the portions 7 and 12. The part 6 is then withdrawn again by means of the hand wheel 10. During this withdrawal friction between the inside of the portion 7 and the mount 18 is suflicient to pull the said mount from its spring holder. This mount will now be inside portion 7. Another mount 18 is now brought into position between the envelope portions 7 and 12, the part 6 is again reciprocated and this further mount is transferred into the envelope portion 7. This transference of selected photo-cathodes on the mounts 18 from the wheel to the envelope portion 7 continues until the desired number of photo-cathodes and fluorescent screens is within the said envelope port-ion 7. It will be seen that the spacing of the membranes 17 in the envelope portion 7 will be determined by the lengths of the cylinders 18.

The wheel and screw mechanism 10-11 is now removed leaving the chamber still sealed. A portable hydraulic press (not shown) is lowered over the chamber and pressure is applied between the parts 19 and 20 which are formed to constitute thrust blocks for taking the pressure of the hydraulic press. Also the wheel 15 is rotated until a cut-out portion in the rim thereof is brought between the envelope portions 7 and 12. These two portions are now brought into contact (the cut-out portion of the wheel 15 permitting this) by the hydraulic press and sufficient pressure is applied between them to cause a cold weld to occur between the copper flanges 7 and 12. The envelope is thus sealed. Vacuum is then released from the chamber, the lid 1 taken off the base 2 and the sealed envelope removed.

In order to obtain a good vacuum in the envelope, it is usually advisable to getter it in accordance with known practice. In such cases a getter may be conveniently provided on the edge of the envelope closure portion 12 or on the inside of an extra glass mount 18 having no membrane and which is inserted in the envelope portion 7 as the last operation before bringing the portions 7 and 12 together and sealing them.

If desired, the mounts 18 may be used not only as mechanical supports and spacers for the membranes, but may be coated on the inside with conducting coatings or spiral-s so as to provide a uniform potential gradient between one membrane and the next when the image amplifier is in use. Of course, external connectors (not shown) to the membranes and passing through the envelope portion 7 may be provided as required.

In the case of image amplifiers of high gain there is a possibility that X-rays caused by electrons striking the final fluorescent screen of the tube may be absorbed in the first photo-cathode and produce dark current photoelectrons and, in extreme cases, this effect may be regenerative and result in loss of picture. Trouble from X-rays may be greatly reduced and in practice eliminated by making the membranes of lead glass.

An advantage of the apparatus of this invention is that the provision of the novel means for individual treatment of individual photo-cathodes Without affecting other photo-cathodes and without difficulties due to liberated caesium, permits the manufacture of a high gain image amplifier with the successive photo-cathodes spaced apart by much smaller amounts than are usually regarded as necessary. Indeed, it may be possible to adopt spacings as low as only a few millimetres each. While this obviously reduces the size of the amplifier, it also has the greater advantage that it enables focusing on the successive fluorescent screens to be obtained without the use of externally applied magnetic fields and the consequent elimination of solenoids or magnets for such fields not only reduces cost, but greatly reduces weight an important matter for airborne apparatus in particular.

In some cases it is desired that the first cathode of a cascade image amplifier tube shallbe deposited direct on one of the end walls of the envelope and the final fluorescent screen deposited direct on the other end wall of said envelope. Such a tube is represented in FIG- URE 1 and the present invention can be used in the manufacture of such tube. FIGURE 3 shows, so far as is necessary for an understanding thereof, a modification of the novel apparatus of FIGURE 2 which permits this method of manufacture to be done. Where such a tube is to be made the portion 7 of the envelope (FIGURE 3) is provided with a preformed fluorescent screen 21 deposited on the inside of its end wall and the end closure portion 22 of the envelope is mounted in a holder on the Wheel 15. The spacer 22 does not carry a membrane and is placed in the tube 7 prior to its insertion in the tube 6. After a photo-cathode has been formed on the inside of the end wall of saidportion 22 while it is on the wheel and after the selected membrane mounts have been put into the envelope portion 7, the wheel is turned to bring the portion 22 opposite the portion 7. The part 6 is again reciprocated and the closure portion 22 is transferred from its spring mount into the portion 7 where it is held by friction and the tube is sealed as already described. Obviously, when a tube is to be assembled in this way the membrane mounts must be placed in their holders on the wheel the other way round so that, in the finished tube, each photo-cathode faces a fluorescent screen. Preferably the Wheel 15 carries more than one closure portion 22 and that one having the best photocathode is selected for use.

In utilizing this invention, there is provided an external oven (not shown in the drawings) which is big enough to contain the whole evacuatable chamber of FIG- URE 2 and which is raised to a temperature of several hundred degrees during the initial evacuation. This facilitates the obtaining of a high degree of cleanliness and enables a really high vacuum to be attained during processing. All the materials used in this case must be such as to withstand this temperature and such as will outgas properly. It is also of advantage to provide getters in the evacuatable chamber and to fire them immediately before cathode processing is commenced, thereby removing residual gas traces which might otherwise harm the cathodes. Provision is also preferably made for cold traps in the pipe line from the evacuatable chamber to the vacuum pump, the upper of which can be baked at the same temperature as the chamber so as to aid in securing a high vacuum during cathode processing.

This application is a divisional application of my application Serial Number 2,648, filed January 15, 1960.

What is claimed is:

1. Apparatus for manufacturing an image amplifier comprising evacuatable chamber means, wheel means mounted in said chamber and constituting a carrier member adapted lightly to hold a plurality of membranes and at least one envelope closure portion in a circle concentric with the axis of said wheel means, an oven mounted in said chamber and positioned in the path of rotation of the wheel means, said wheel means including means for moving said membranes and said closure portion in succession through said oven, means in said chamber for holding a main envelope portion in spaced relationship with and adjacent the wheel means, means for reciprocating the main envelope portion toward and away from said wheel means to transfer said membranes to said envelope portion in turn and finally said closure portion to the main envelope portion, and means for tightly pressing together said main envelope portion and said envelope closure portion to cold seal the adjacent faces of the main envelope portion and the envelope closure portion.

2. Apparatus for manufacturing an image amplifier comprising evacuatable chamber means, wheel means mounted in said chamber and constituting a carrier member adapted lightly to hold a plurality of membranes and at least one envelope closure portion in a circle concentric with the axis of said wheel means, an oven mounted in said chamber and positioned in the path of rotation of the wheel means, said wheel means including means for moving said membranes and said closure portion in succession through said oven, means in said chamber for holding a main envelope portion in spaced relationship with and adjacent the wheel means, means for producing relative motion between said wheel means and said main envelope portion to transfer said membranes and said closure portion to said main envelope portion, and means for tightly pressing together said main envelope portion and said envelope closure portion to cold seal the adjacent faces of the main envelope portion and the envelope closure portion.

3. Apparatus according to claim 2 wherein said oven defines an encircling enclosure having a passage therethrough to receive said membranes and said envelope closure portion.

4. Apparatus for manufacturing an image amplifier comprising evacuatable chamber means, supporting means in said chamber for supporting a plurality of membranes and an envelope closure portion, said supporting means including wheel means mounted in said chamber and constituting a carrier member adapted lightly to hold said membranes in a circle concentric with the axis of said wheel means, an oven mounted in said chamber and positioned in the path of rotation of the wheel means, said wheel means including means for moving said membranes in succession through said oven, means in said chamber for holding a main envelope portion in spaced relationship with and adjacent the wheel means, means for producing relative motion between said wheel means and said main envelope portion to transfer said membranes to said main envelope portion, and means for tightly pressing together said main envelope portion and said envelope closure portion to cold seal the adjacent faces of the main envelope portion and the envelope closure portion.

No references cited. 

